1. Field of the Invention
The present invention relates to single-to-differential amplifiers, and more particularly to a high-efficiency single-to-differential amplifier with current reuse.
2. Description of the Prior Art
At present, electronic devices are ubiquitous. They can be found in nearly every place imaginable, including the home, the workplace, vehicles, and even our pockets. And, as electronic device technologies mature, for the same cost, the electronic devices become more portable, use less power, and offer greater functionality. In part, this is due to availability of smaller geometry electronic components, such as transistors and on-chip capacitors. However, novel circuit architectures with improved specifications are also responsible for overall performance gains in the electronic devices.
Amplifiers are a key component in practically every electronic device. They vary broadly in their electrical characteristics, such as gain, bandwidth, and linearity, and vary even more in their application to active filters, buffers, analog-to-digital converters, and RF transceivers.
Different amplifier architectures are necessary for each application. One such amplifier is a single-to-differential amplifier, which is commonly applied in RF circuits as a power amplifier. Please refer to FIG. 1, which is a diagram of a single-to-differential amplifier 100 according to the prior art. The single-to-differential amplifier 100 comprises a first transistor Q1, a second transistor Q2, a first load Z1, a second load Z2, a bias transistor Qb, and a first capacitor C1. The first load Z1 is coupled between a collector of the first transistor Q1 and a first power supply voltage, and the second load Z2 is coupled between a collector of the second transistor Q2 and the first power supply voltage. The first capacitor C1 is coupled between the base of the second transistor Q2 and an AC ground. An emitter of the first transistor Q1 is coupled to an emitter of the second transistor Q2 and a collector of the bias transistor Qb. An emitter of the bias transistor Qb is coupled to a second power supply voltage. A base of the first transistor Q1 takes a single-ended input signal, and the collectors of the first transistor Q1 and the second transistor Q2 output a differential output signal.
In the single-to-differential amplifier 100, a bias current I flows through the first transistor Q1 and the second transistor Q2. Thus, the bias transistor Qb provides a bias current 21 that is double the bias current I. The single-to-differential amplifier 100 is not particularly efficient, since the bias transistor Qb only acts as a current source to provide DC current, and does not provide amplification. Thus, the three transistors Q1, Q2 and Qb form a “one stage” single-to-differential amplifier.
Please refer to FIG. 3, which is a diagram of a single-to-differential low noise amplifier (LNA) according to the prior art. The single-to-differential LNA shown comprises two transistors 301, 302 biased by two current sources 303, 304 connected to voltage supply Vcc, and two current sources 309, 310 connected to ground. A resistor 320 connects emitters of the two transistors 301, 302 as a feedback element. The LNA takes an input at a node INH, and is coupled to AC ground at a node LMD through a capacitor 322. Amplifiers 305, 306 amplify outputs from collectors of the two transistors 301, 302, respectively, and resistors 307, 308 provide negative feedback to ensure constant current at the transistors 301, 302. Use of the resistor 320 and the resistors 307, 308 improves noise performance, and also allows for lower bias current requirements for the two transistors 301, 302.
Please refer to FIG. 4, which is a diagram of a second single-to-differential amplifier 40 according to the prior art. The amplifier 40 comprises a bias circuit 419, which establishes bias currents in a transistor 432 and a transistor 422. A voltage-to-current element 425 converts an input voltage received at a node 423 to an input current. The input current causes differential variations in the currents in the transistor 432 and the transistor 422, which are outputted by diodes 436, 438. The amplifier 40 generates a differential output signal.
The prior art provides a variety of single-to-differential amplifiers, but none is able to provide high efficiency to meet today's technological demands.